1. Field of the Invention
The present invention generally relates to a non-volatile memory device and manufacturing method thereof, and more particularly, to a non-volatile memory having a composite type of charge-trapping layer and method for manufacturing the same.
2. Description of Related Art
Electrically programmable and erasable non-volatile memorizing techniques based on charge storage structure such as electrically erasable programmable read-only-memory and flash memory have multiple modern applications.
With the miniaturization of integrated circuits, charge-trapping layer based memory unit structure has become more and more critical due to the measurability and simplicity of the manufacturing process. The charge-trapping layer based memory unit structure includes, for example, nitride trapping layer memory, silicon-oxide-nitride-oxide-silicon (SONOS) and Bandgap-Engineered silicon-oxide-nitride-oxide-silicon (BE-SONOS). These memory unit structures store data by trapping charges in the charge-trapping layer (for example, a silicon nitride layer).
Moreover, the conventional non-volatile memory mainly uses a polysilicon material layer as a charge storage layer (that is, a floating gate). Once the tunnel oxide layer produces a leakage path, all the stored electric charges will leak away. This is a big challenge to the characteristics, reliability and tolerance of the device. Therefore, a silicon-oxide-nitride-oxide-silicon (SONOS) structure has been developed to deal with the problem. Because the nitride layer is rich with charge storage centers, it can be used for charge storage. Moreover, because the nitride layer is a dielectric layer, lost of charges as a result of a leakage path in the tunnel oxide layer can be prevented.
If the silicon nitride of the silicon nitride layer in the SONOS structure is a nitrogen-rich silicon nitride film, the silicon nitride layer has mostly deep energy level charge storage centers so that the charge trapping density is poorer compared with the shallow energy level defects. However, if the silicon nitride is an silicon-rich silicon nitride film, the silicon nitride layer has mostly shallow energy level charge storage centers. Although the charge trapping density is higher, the leaking rate is also higher and can lead to very low charge storage ability. Furthermore, the thermal budget of the conventional method for forming the SONOS is too high and potentially impact on the device performance. Therefore, by adjusting the atomic composition and content of the nitride layer, the characteristics of the device including its speed, reliability and tolerance can be affected.